CODEWITHSUNDEEP
javaperformance
Julias
Julias

Reputation: 5892

Java hash utilities

I'm looking for a good hash utility or framework to map the string to some int value in range say {1 .. k} I should use it in the runtime component that receives a lot of concurrent requests from users with 'userId' that is string and i want to map that value to defined integer range.

Of course the length of the userId value could be different but it is ok with me to assumt on some minimum lenght for using this function

It should be very fast

Thanks

Upvotes: 2

Views: 1794

Answers (4)

John Eipe
John Eipe

Reputation: 11236

You could go ahead with HashTable or HashMap that comes with JDK. Choosing between the two, goes down to this,

  • access to the Hashtable is synchronized on the table while HashMap isn't.

  • Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.

  • HashMap permits null values in it, while Hashtable doesn't.

HashMap is part of the new collections framework (since JDK 1.2)

If you have millions of entries go for a database. Go for NoSQL.

Similar question here HashMap in Java, 100 Million entries

Upvotes: 0

Peter Lawrey
Peter Lawrey

Reputation: 533820

You can use String.hashCode(). 

String a1 = "Hello World";
String a2 = new String(a1); // don't do this unless you have to have a different object.
System.out.println("Identity hashCode " + System.identityHashCode(a1) + " != " + System.identityHashCode(a2));
System.out.println("String.hashCode " + a1.hashCode() + " == " + a2.hashCode());

prints

Identity hashCode 551677275 != 1353056826
String.hashCode -862545276 == -862545276

In terms of performance, hashCode() is much faster than creating the String itself. If this is not fast enough, I would avoid using/creating String in the first place. (highly unlikely you need to so this)

The identity hash code changes each time the program is run. Note: hashCode() can be negative so you have to adjust for this.

int hash = (text.hashCode() & 0x7FFFFFFF) % K + 1;

or if you don't want to discard the top bit

int hash = (int) ((text.hashCode() & 0xFFFFFFFFL) % K + 1);

Upvotes: 1

Adam
Adam

Reputation: 36743

If you're after a secure hash (one that cannot be reversed easily), then use message digest:

try {
    MessageDigest msgDigest = MessageDigest.getInstance("MD5");
    byte digest[] = msgDigest.digest(username.getBytes());
    int secureHash = 1 + new BigInteger(digest).mod(BigInteger.valueOf(k)).intValue();

    System.out.println("Secure hash " + secureHash);
} catch (NoSuchAlgorithmException e) {
    e.printStackTrace();
}

Upvotes: 0

Yann Ramin
Yann Ramin

Reputation: 33197

Every Java object has a builtin hashCode method, which returns an int. For String it is pre-defined for you (it needs to be implemented for custom objects).

To map this to 1..k, where k is an integer, consider using the modulus:

String hi = "Hello";
int hash = (hi.hashCode() % K) + 1;

Upvotes: 3

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